1. Field of the Invention
This invention applies to the technology of food processing, and particularly the dehydration of fruits and vegetables. The invention is of particular value in the processing of prunes.
2. Description of the Prior Art
The history of commercial-scale dehydrators for prunes and other foodstuffs begins with natural draft dehydration. This process involved placing the prunes on wooden trays and stacking the trays near a hillside. A fire was lit at the bottom of the hillside to produce a natural draft and the smoke and air passing through the stack drove moisture from the prunes and carried the moisture off as water vapor. Methods of greater sophistication were eventually introduced, notably the use of a box-like dryer in which heat was produced artificially by oil and a large fan forced the heated air across the prunes. Humidity shutters and bleeder vents controlled the flow of moisture entrained by the heated air.
The dehydrators in current commercial use are tunnel dehydrators. Although these dehydrators vary considerably in design, they typically receive product spread on trays which are stacked and placed on wheeled dollies or cars which are advanced through the tunnel where they are exposed to heated air. Each tray has end and center cleats to provide a clearance of about 2.5 inches (about 6 cm) between adjacent trays in a stack, and a car and its tray stack together have a height of approximately 6.5 feet (about 2 m). The dehydrator tunnel accommodates a row of loaded cars, and once placed inside the tunnel, the loaded cars are advanced through the tunnel by rams or other similar conveyors. Each car moves from one end of the tunnel to the other while heated air is passed through the tunnel across the loaded cars. Conveyor belts have also been used in place of the trays and cars.
In the typical drying of prunes that are spread on tray stacks loaded onto cars, the cars move through the tunnel in increments, each increment advancing the cars one car length with the cars remaining stationary for approximately two hours between increments. The passage of air through the tunnel occurs continuously both while the cars are stationary and while they are moving, while the burners are idled down when the cars are ready to be moved. When the cars are moving, the tunnel is opened at both ends to allow a car with fully dehydrated product to be removed from one end and a fresh car with wet product inserted at the other end. Even though the cars are stationary during the air flow, the direction of air flow through the tunnel is a factor in the dehydration efficiency since the moisture level of product in any single car as the heated air flow begins is dependent upon the time that the car has spent in the tunnel which in turn depends on how far the car has advanced in its travel through the tunnel. The dehydration effect of the heated air also depends on the moisture level in the air, which varies with the distance that the air has traveled through the tunnel before reaching any particular car. Thus, dehydration tunnels in which the air flow and the advancement of cars through the tunnel occur in opposite directions are characterized as “counter flow,” while those in which the air and the cars move in the same direction are characterized as “parallel flow,” even though the cars are stationary while the air is actually flowing over them.
Counter-flow dehydration is the type most commonly used for raisins, and the typical number of cars in a row inside the tunnel is eight to ten cars. Both single-lane and twin-lane tunnels are used. The air at the end at which the cars leave is dry, hot and relatively high in pressure, while the air at the end of the tunnel at which the incoming cars is relatively cool and low in pressure. With each step forward, therefore, a car is exposed to dryer and hotter air until the car is removed from the tunnel.
Parallel flow dehydration is also used, however, particularly in installations having a high throughput rate of product. With both the product and hot air entering the tunnel at the same end, the drying rate at the entry end of the tunnel is very high. Also, product with the highest moisture content is exposed to the warmest air, and the evaporative cooling of the product at this location in the tunnel keeps the product considerably cooler than the air. Parallel flow dehydration is typically used when production requirements outweigh quality concerns.
Among the concerns in dehydration are carmelization, cooking, and case hardening. Carmelization is the burning of sugars in the product and occurs when the temperature and air velocity in the dehydrator are too high. Carmelized product is unfit for commercial sale and cannot be salvaged. “Cooking” is chemical transformation of the oils and sugars in the product due to excessive heat. Although cooked product is not unfit for consumption, the product cannot be returned to an uncooked state. Case hardening, which is likewise caused by excessive temperatures and air velocities but also by low humidity, is the formation of a tough, leather-like outer skin on the product which reduces the ability of moisture to escape the product. Case hardened product can often be salvaged, but only by massive re-hydration.
Parallel flow dehydration is particularly vulnerable to each of these undesirable effects, since the introduction of a car with fresh wet product upstream raises the moisture level in the air flowing through the tunnel. Thus, product in the car immediately downstream of the newly introduced car, which was partially dehydrated before the new car was introduced, is rehydrated with moisture from the new car. All cars in the tunnel are similarly affected, and the result is temperature cycling in all cars and in the air contacting the cars, each cycle initiated with each introduction of a fresh car. The cycling repeatedly exposes the product to relatively high temperatures which remain high as the moisture level drops and the product is increasingly vulnerable to damage from the excess heat.